Two-stage closed center electro-hydraulic valve

ABSTRACT

A servovalve ( 15 ) comprising a motor ( 16 ), a motor bias mechanism ( 20 ), a first stage valve member ( 22 ) adapted to be moved from a first position to a first off-null position, a second stage member ( 29 ) adapted to be moved from a first position to a second position with movement of the first valve member ( 22 ), a transfer link ( 34 ) acting between the first ( 22 ) and second ( 29 ) valve members, an eccentric drive member ( 35 ) acting between the motor ( 16 ) and the transfer link ( 34 ), the transfer link ( 34 ) and drive member ( 35 ) configured such that selective movement of the motor ( 16 ) causes the transfer link ( 34 ) to move the first valve member ( 22 ), movement of the first valve member ( 22 ) causes the second valve member ( 29 ) to move, and movement of the second valve member ( 29 ) causes the transfer link ( 34 ) to move the first valve member ( 22 ) from the first off-null position back to the null position.

TECHNICAL FIELD

The present invention relates generally to the field of electrohydraulicservovalves and, more particularly, to an improved two-stageelectrohydraulic servovalve.

BACKGROUND ART

Electrohydraulic servovalves are known. These may be thought of ashaving either a single stage or as having multiple stages. In bothforms, a valve spool is slidably mounted within a cylinder forcontrolled movement there along. When the valve spool is in a centeredor null position within the cylinder, various lands on the valve spoolcover ports that communicate with the control outlets to prevent flowthrough the valve. The direction and magnitude of spool movementoff-null controls the flows through the valve. Various forms ofsingle-stage servovalves are representatively shown and described inU.S. Pat. No. 4,951,549, U.S. Pat. No. 5,263,680, U.S. Pat. No.4,641,812, and U.S. Pat. No. 5,146,126, the aggregate disclosures ofwhich are hereby incorporated by reference.

A single-stage or direct-drive valve generally has a motor or some othermechanism that directly engages the valve spool, and which selectivelycauses the spool to move off-null. A multiple-stage valve may have apilot stage that controls movement of a valve spool in a second stage.The pilot stage may be an electrical section, and the second stage maybe an hydraulic section. One example of a two-stage electrohydraulicservovalve is shown and described in U.S. Pat. No. 3,228,423, theaggregate disclosure of which is hereby incorporated by reference.

BRIEF SUMMARY OF THE INVENTION

With parenthetical reference to the corresponding parts, portions orsurfaces of the disclosed embodiment, merely for purposes ofillustration and not by way of limitation, an improved two stageelectro-hydraulic servovalve (15) is provided comprising a motor (16)having a stator (18) and a rotor (19) having a rotor null position(FIG. 1) and configured and arranged to rotate about a motor axis (17)under the effect of a magnetic field generated by the stator; a biasmechanism (20) configured and arranged to bias the rotor to the rotornull position; a first stage valve (21) having a first valve member (22)movably mounted in a first chamber (23) along a first valve axis (24),and adapted to be moved from a first null position (FIG. 1) to a firstoff-null position (FIG. 4) along the first chamber axis to selectivelymeter fluid flow from at least one port (25, 26) defined between thefirst valve member and the first chamber; a second stage valve (28) influid communication with the port of the first valve and having a secondvalve member (29) movably mounted in a second chamber (30) along asecond valve axis (31), and adapted to be moved from a first position(FIG. 1) to a second position (FIG. 5) along the second valve axis as afunction of movement of the first valve member, to selectively meter theflow of fluid from at least one port (32, 33) between the second valvemember and the second chamber; the first stage valve and the secondstage valve configured and arranged such that the second stage valvemember is at a pressure equilibrium and does not move when the firststage valve member is in the null position; a transfer link (34) actingbetween the first valve member and the second valve member; an eccentricdrive member (35) acting between the rotor and the transfer link andhaving a first eccentric axis (36) that is off-set a distance (51) fromthe motor axis and arranged such that selective rotation of the rotorabout the motor axis causes the transfer link to move; the transfer linkand the drive member configured and arranged such that selectivemovement of the rotor from the rotor null position to a second rotorposition (FIG. 4) causes the drive member and the transfer link to movethe first valve member from the first null position to the firstoff-null position (FIG. 4); movement of the first valve member from thefirst null position to the first off-null position causes the secondvalve member to move from the first position to the second position(FIG. 5); and the movement of the second valve member to the secondposition causes the transfer link to move the first valve member fromthe first off-null position back to the null position (FIG. 5).

The transfer link may comprises a first end portion (58) engaging thefirst valve member at a first connection (70); the transfer link maycomprises a second end portion (59) engaging the second valve member ata second connection (72); and the eccentric drive member and thetransfer link be coupled at a third connection (71). The transfer linkand the eccentric drive member may be rotationally coupled at the thirdconnection. The transfer link may be configured and arranged to move thefirst valve member from the first null position to the first off-nullposition with selective rotation about the second connection. Thetransfer link may be configured and arranged to move the first valvemember from the first off-null position back to the null position withselective rotation about the third connection. The transfer link may beconfigured and arranged to move the first valve member from the firstoff-null position back to the null position with selective rotationabout the first eccentric axis. The first eccentric axis (36) may bealigned with the third connection (71). The first stage valve maycomprise a second port (26), the second chamber of the second stagevalve may comprise a first sub-chamber (65 a) and a second sub-chamber(65 b), the port my be flow connected to the first sub-chamber and thesecond port may be flow connected to the second sub-chamber, and thesecond valve member may be adapted to be moved from the first positionto the second position along the second valve axis as a function of ahydraulic pressure differential between the first sub-chamber and thesecond sub-chamber. The bias mechanism may comprise a torsional spring(46). The rotor may consists essentially of a magnet. The stator maycomprise a circular ring-like core (43) and windings about the core (44,45) orientated in opposite directions around the core. The first chamberand the second chamber may each comprise a cylinder and the first stageand the second stage valve members may each comprise a valve spool. Thefirst stage valve member may comprise a slot (75) bounded bysubstantially-parallel walls (60) and the transfer link may comprises arounded marginal end portion (58) engaging the slot walls. The secondvalve member may comprise a slot (76) bounded by substantially-parallelwalls (61) and the transfer link may comprise a second rounded marginalend portion (59) engaging with the slot walls. The servovalve maycomprise at least one bearing (56) acting between the drive member andthe transfer link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of an improved two-stageelectro-hydraulic valve, in which the first stage valve is in a centeredor null position relative to the cylinder and the second stage valve isin a first position that prevents flow through the second stage valve.

FIG. 2 is an enlarged schematic view of the motor shown in FIG. 1.

FIG. 3 is a vertical cross-sectional view of the valve shown in FIG. 1.

FIG. 4 is a schematic view of the valve shown in FIG. 1, in which therotor has been rotated about 10° in a clockwise direction from theposition shown in FIG. 1, such movement producing concomitant movementthrough the drive member and the transfer link of the first stage valvespool off-null.

FIG. 5 is a schematic view of the valve shown in FIG. 1, in which thesecond stage valve has moved to the desired second position, suchmovement producing through the transfer link concomitant movement of thefirst stage valve spool back to the null position shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., crosshatching, arrangement of parts, proportion,degree, etc.) together with the specification, and are to be considereda portion of the entire written description of this invention. As usedin the following description, the terms “horizontal”, “vertical”,“left”, “right”, “up” and “down”, as well as adjectival and adverbialderivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”,etc.), simply refer to the orientation of the illustrated structure asthe particular drawing figure faces the reader. Similarly, the terms“inwardly” and “outwardly” generally refer to the orientation of asurface relative to its axis of elongation, or axis of rotation, asappropriate.

Referring now to the drawings, an improved two-stage electro-hydraulicvalve is provided, an embodiment of which is generally indicated at 15.Valve 15 is shown as broadly including motor 16, bias mechanism 20,drive member 35, transfer link 34, first stage valve 21, and secondstage valve 28.

In this embodiment, motor 16 is a rotary brushless DC toroid motorhaving a single pole and phase and limited angular displacement. Asshown, motor 16 includes stator 18 and rotor 19. As shown in FIG. 2, inthis embodiment stator 18 is a toroid and has a circular ring-shapedcore 43 around which coils 44 and 45 are wound. Coil 44 is woundclockwise half-way around core 43, from the six o'clock position to thetwelve o'clock position, and is then looped, not wound, back to the sixo'clock position, while coil 45 is wound counterclockwise half-wayaround core 43, from the six o'clock position to the twelve o'clockposition, and is then looped, not wound, back to the six o'clockposition. In addition, coils 44 and 45 are wound around core 43 inopposite directions relative to the center circular axis 17 of core 43.Coil 44 is wrapped in one direction and, depending upon the rotation andthe magnetic field, current will produce an electromagnetic force in onedirection. By bringing the end of coil 44 back around and connecting itover to coil 45, which is wrapped around core 43 in the oppositedirection, torque summing from both halves is provided but theinductance is almost cancelled. With the wiring shown, having a patharound one half of the circumference of metallic soft magnetic core 43,and a second path around the other half of the circumference of core 43,wrapping around and starting on the opposite end and then coming back,torque summation from current and electromagnetic force is provided butinductance is canceled. And by joining the wires as shown, a single coilis provided. However, to impart redundancy, wires 44 and 45 may betwo-stranded wires or twisted pairs. If power from a controller or acomputer is lost in one wire, or if one of the wires breaks, the otherwire is available to do the same exact thing. Electrically this resultsin a very fast device.

By wrapping the coils 44 and 45 around ring-like core 43 to provide atoroid, the torque density as compared to a stator toothed design isless. However, there is also significantly less cogging or torqueripple. In addition, by wrapping wires 44 and 45 around soft magneticcore 43 and changing current in both directions, depending upon thefrequency or speed of current that is flowing through the wire, eddycurrent losses are produced that act like little dampers or resistanceto motion as a function of speed, or increased torque resistance as afunction of speed. The typical manner of handling that is to form statorcore 43 from laminates. A core having multiple laminates and insulatingmaterial there between can be used to reduce or eliminate such dampeningor eddy current loss. However, in this embodiment, some dampening isdesired. By designing stator core 43 with a certain number of laminates,such as two halves, three-thirds, four-fourths or more, the amount ofdampening can be selected to provide a stable high performance valve.

In this embodiment, rotor 19 consists essentially of a permanent magnet.Thus, other than grounding spring 46 and output shaft 52, the magnetcomprises the entire rotor. The power coming out of the magnet is moresubstantial and, because of the shape of the magnet, the arc angle ofthe magnet can be larger without causing manufacturing complications.

As shown, bias mechanism 20 acts on rotor 19. In this embodiment, biasmechanism 20 comprises torsional spring 46 and clamp 48 set to biasrotor 19 back to a desired null or grounded position. Mechanism 20 notonly acts like a spring to bias rotor 19, it also supports or holdsrotor 19 in position relative to stator 18. Rather than rotating aboutaxis 17 on bearings, rotor 19 is suspended by spring 46 and acts as aflexural straining element. This eliminates parts like bearings andfriction elements. As shown, spring 46 is part of rotor 19 and extendsto the top of the cavity of upper part 41 of housing 38, where it isgrounded. A single jewel bearing 49 at the bottom of rotor 19 is used toremove side motion. To reference rotor 19 and bias mechanism 20 to zeroor ground, so that motor 16 and valves 21 and 28 are grounded andreferenced to housing 38, clamp 48 can be adjusted or rotated relativeto housing 38 to get the position desired to reference the null of motor16 relative to the output flow of second stage valve 28. Clamp 48 can berotationally adjusted, and thus rotor 19 rotationally adjusted relativeto stator 18, to provide a desired motor null position in which torsionspring 46 is not flexed or strained. Thus, the null position of rotor 19may be adjusted as desired by the user by resetting clamp 48 to thedesired rotational position without having to access motor 16. While atorsional spring is shown and described, other bias mechanisms may beused as alternatives.

As shown, output shaft 52 of rotor 19 rotates about motor axis 17relative to housing 38. In this embodiment, drive member 35 is aneccentric cylindrical shaft elongated about center axis 36. Eccentricdrive shaft 35 is rotationally coupled to rotor 19 by offset link 50,such that motor axis 17 is offset a distance 51 from center longitudinalaxis 36 of drive shaft 35. Thus, output shaft 52 of rotor 19 is linkedto eccentric drive 35 such that longitudinal axis 36 of eccentric driveshaft 35 is set off from rotational axis 17 of rotor 19. When rotor 19rotates about axis 17, that rotation is transferred to eccentric driveshaft 35 and causes distal end 54 of shaft 35 to move in an arc path 53.Such motion is transferred to transfer link 34 at connection joint 71.

Transfer link 34 extends generally transversely between spool 22 ofvalve 21 and spool 29 of valve 28. As shown, transfer link 34 comprisesfirst end 58 that engages slot or seat 75 in spool 22 at connectionjoint 70, second end 59 that engages slot or seat 76 in spool 29 atconnection joint 72, and recess or opening 55 between ends 58 and 59sized and configured to receive end pin 54 of eccentric drive shaft 35to form connection joint 71. As shown, end 58 is a rounded ball-like endportion that is received between two facing parallel planar walls,severally indicated at 60, of notch or slot 75 in valve spool 22.Similarly, end 59 is a rounded ball-like end portion that is receivedbetween two facing parallel planar walls, severally indicated at 61, ofnotch or slot 76 in valve spool 29. This design is such that transferlink 34 takes out possible misalignment between spools 22 and 28 androtor axis 17. Since ends 58 and 59 of transfer link 34 are generallyspherical and machined into link 34, they allow transfer link 34 toangularly sweep and still drive valve spools 22 and 28, but they alsoallow the system to dynamically adjust for misalignment and therebyreduce the potential for binding, excessive friction and jamming. Link34 allows the spools and drive shaft to seek their defined positionssuch that all of the members are mechanically connected but not binding.

End pin 54 of eccentric drive 35 fits within hole 55 in transfer link34. Annular earing 56 between pin 54 of drive 35 and hole 55 of transferlink 34 allows for some relative rotational movement at connection joint71. However, movement of pin 54 of drive 35 in arc 53 causes transferlink 34 to move in what appears to be a linear fashion, thereby movingfirst spool 22 along axis 24.

As shown in FIG. 3, valve 15 is an assembled body, generally indicatedat 38. Body 38 includes lower or base part 39 housing first stage valve21 and second stage valve 28, intermediate or central part 40 housingmotor 16, and upper or top part 41 housing bias mechanism 20. Thus,inside body 38 of valve 15 are two spools 22 and 29 that sit in chambers23 and 30, respectively, machined into bushings that are pressed intobody 38.

Four ports come into body 38. As shown in FIGS. 1 and 3, base 39 ofvalve 15 has operative connections to supply pressure Ps, fluid return Rand two control ports, C1, C2, respectively. Hence, because there arefour fluid connections, this valve is a four-way servo valve. However,it should be clearly understood that the embodiments are not limited tofour-way valves, but could be readily adapted to a three-way valve, orsome other form, as desired. Control ports C1 and C2 are the output ofsecond stage valve 28. Supply port Ps brings in high pressure oil, wateror other fluid or gas and connects to both supply or pressure chambers63 a and 63 b of second stage valve 28 as well as pressure chambers 62 aand 62 b of first stage valve 21. Because first valve 21 is a pilotstage and has slots that are very small, filters 64 a and 64 b areprovided in the supply lines to pressure chambers 62 a and 62 b,respectively, to trap and contain any particles of contamination andprevent valve spool 22 from jamming. The output flow from ports 25 and26 of first stage valve 21 connect to the respective end chambers 65 aand 65 b of chamber 30 of second stage valve 28.

Base 39 has two horizontal through-bores which form chambers 23 and 30to receive and accommodate sliding movement of valve members 22 and 29of first stage valve 21 and second stage valve 28, respectively. In thisembodiment, chambers 23 and 30 are cylindrical. However, suchthrough-bores may have non-circular cross sections, causing the chambersto be a non-cylindrical shape, such as a rectangular prism, or othersimilar shape. In this embodiment valve members 22 and 29 arecylindrical valve spools. However, the valve spools may have alternativeshapes, such as a rectangular prism forming a shear plate. Valvechambers 23 and 30 and spools 22 and 29 are elongated about axis 24 and31, respectively, such that valve spool 22 moves linearly along axis 24and valve spool 29 moves linearly along axis 31, which is parallel toaxis 24. Both axis 24 and axis 31 are transverse to motor axis 17 andlongitudinal axis 36 of eccentric drive shaft 35.

Base 39 also includes a horizontal through bore extending transverselybetween chambers 23 and 30 which forms chamber 42 to receive andaccommodate movement of transfer link 34 acting between first stagespool 22 and second stage spool 29. Intermediate part 40 is adapted toface and engage base part 39 and houses motor 16. Upper part 41 is inthe nature of a cover which protectively surrounds and covers mechanism20.

As shown in FIGS. 1 and 3, valve spool 22 comprises a plurality of landsand grooves along its longitudinal extent in the usual manner, and isadapted to be selectively and controllably shifted by end 58 of transferlink 34 either leftwardly or rightwardly, as desired, within cylinder 23from the null position along axis 24 shown in FIG. 1. In this nullposition, respective lands on valve spool 22 cover ports 25 and 26communicating with chambers 65 a and 65 b, respectively, of cylinder 30of second valve stage 28. As shown, in the null configuration of FIG. 1,hydraulic flow between hydraulic supply Ps and supply chamber 62 athrough port 25 of cylinder chamber 30 is blocked by lands 68 b.Similarly, hydraulic supply Ps and supply chamber 62 b through port 26of cylinder chamber 30 is blocked by lands 68 c. Hydraulic fluid inchambers 62 a and 62 b is prevented from flowing out by spool lands 68 band 68 c, respectively. Thus, spool 22, and in turn spool 29, areconstrained from moving due to pressure equilibrium.

As shown in FIGS. 1 and 3, valve spool 29 comprises a plurality of landsand grooves along its longitudinal extent in the usual manner, and isadapted to be selectively and controllably shifted by differentialpressure between end chambers 65 a and 65 b either leftwardly orrightwardly, as desired, within cylinder 30 from the position along axis31 shown in FIG. 1. In this position, respective lands on valve spool 29cover ports 32 and 33, respectively, of control openings C1 and C2 toprevent flow through the valve.

Coil 44, 45 may be selectively energized by supplying it with a currentof appropriate magnitude and plurality to cause rotor 19 to rotate aboutaxis 17 in either a clockwise or counterclockwise direction. Thedirection of rotor movement is determined by the polarity of thesupplied current. The magnitude of angular rotor movement is determinedby the magnitude of the supplied current.

In FIG. 4, rotor 19 is shown as having just rotated about axis 17approximately 10° in a clockwise direction from the rotary null positionshown in FIG. 1. When rotor 19 rotates clockwise about axis 17, as shownin FIG. 4, such rotation causes pin 54 of eccentric drive shaft 35 tomove to the right along arc 53. Because at this point spool 29 isconstrained from movement due to equalized pressure at both its ends asdescribed above, connection joint 72 between ball end 59 of link 34 andspool notch walls 61 of spool 29 momentarily acts as a fixed axis.Because of this and the eccentric offset described above, movement ofpin 54 of eccentric drive shaft 35 to the right along arc 53 causes ballend 58 of transfer link 34 to move to the right. Thus, ball end 58 andconnection joint 70 rotates clockwise relative to connection joint 72.As this occurs, ball end 58 causes valve spool 22 to move in one axialdirection along axis 24 to the right within cylinder 23. As shown inFIG. 4, as valve spool 22 is moved off-null and to the right, spoollands 68 b and 68 c are no longer aligned on ports 25 and 26,respectively, which allows fluid to flow to or from ports 25 and 26,respectively, and in turn to and from ports 73 a and 73 b in chamber 30to piston chambers 65 a and 65 b, respectively, of second stage valve28. Such movement of spool 22 exposes port 25 to high supply pressureand exposes port 26 to low return pressure. This displaced condition ofspool 22 enables fluid to flow into chamber 65 a of second stage valve28 from supply and to flow out of chamber 65 b of second stage valve 28to return, thus creating a pressure differential between one end ofspool 29 and the other end of spool 29.

When this happens, since control ports 25 and 26 from first stage valve21 are feeding the ends of spool 29 of second stage valve 28 asdescribed above, spool 29 is moved in one axial direction along axis 31to the right within cylinder 30. As shown in FIG. 5, as valve spool 29is moved to the right, spool lands 69 a and 69 b are no longer alignedon ports 32 and 33, respectively, which allows fluid to flow to or fromports 32 and 33 and controls C1 and C2, respectively. Such movement ofspool 29 exposes port 32 to high supply pressure Ps and exposes port 33to low return pressure R.

The movement of spool 29 to the right also causes movement of transferlink 34. In particular, as shown in FIG. 5, because at this point endpin 54 of drive shaft 35 is held in position by motor 16, connectionjoint 71 between pin 54 and hole 55 in link 34 acts as a fixed axis.With movement of spool 29 to the right, ball end 59 and connection joint72 moves counterclockwise about connection joint 71 and eccentric axis36, causing transfer link 34 to rotate counter-clockwise aboutconnection joint 71 and eccentric axis 36. Counter-clockwise rotation oftransfer link 34 about axis 36 causes ball end 58 of transfer link 34and connection joint 70 to move counterclockwise about connection joint71 and eccentric axis 36 and to the left. Movement of ball end 58 to theleft causes valve spool 22 to move to the left within cylinder 23 untilfirst stage valve 21 returns to the null position. As shown in FIG. 5,as valve spool 22 is moved left, spool lands 68 b and 68 c realign overports 25 and 26, respectively, which stops fluid flow from and ports 25to end chamber 65 a and from port 26 to end chamber 65 b of chamber 30of second stage valve 28. Spool 29 stops moving with the closing ofports 25 and 26 and the return of equilibrium pressure at both ends ofspool 29. Thus, spool 29 cancels the motion of the held position ofrotor 19 and eccentric drive pin 54 and rotates transfer link 34 aboutaxis 36 until it reestablishes null of first stage spool 22 of firststage valve 21.

If the polarity of the supplied current were reversed, rotor 17 wouldrotate counterclockwise about axis 17, with such rotation causing pin 54of eccentric drive shaft 36 to move to the left along arc 53, in turncausing ball end 58 of transfer link 34 to move to the left, therebycausing spool 22 to move left along axis 24 off null to displace spool22 in the opposite direction relative to cylinder 23. Connection joints70, 71 and 72 are said to be floating connections since their axis isnot fixed relative to actuator body 38. Axes 17 is not floating.

Because rotor 19 is an inertial mass suspended on torsion spring 43, thefrequency of rotor 19 is a potential issue, particularly if thatfrequency is in the middle of the operational frequency of valve 15. Toaddress this, some dampening is provided. It is acceptable to havedamping and slow the response because of the amplification between firststage valve 21 and second stage valve 28. Such dampening is provided intwo places. As mentioned above, some dampening may be provided bycontrolling the number of laminations forming core 43. Second, as shownin FIG. 1, narrowing orifices 74 a and 74 b are provided at the end ofthe fluid connection between R and the end chambers of first stagechamber 23 to help squelch the motion of first stage spool 22. So ifrotor 19 were to start resonating violently, first stage spool 22 wouldhave to start moving with it because spool 22 and rotor 19 are connectedvia drive member 35 and transfer link 34 as described above. If thisstarts to happen, orifices 74 a and 74 b will start to impede the motionof first stage spool 22, making it look like a dynamic attenuator orspring.

Bias mechanism 20 is provided so that valve 15 will have a given ratedflow at a rated current. Spring 46 is selected such that with theappropriate amount of rotational motion of rotor 19, spring 46 willdeflect that same amount and produce the amount of counter torque equalto the rated current and torque constant of motor 16. So if the ratedcurrent is 35 milliamps and 10 degrees of rotation on rotor 19 isdesired, spring 46 is selected accordingly. Thus for a given flow rateoutput there is a given current input to produce it.

Valve spool 29 arrives at a position or a command and mechanicallyconveys that it has arrived at that position. The position of secondstage spool 29 is slaved to first staged spool 22 via transfer link 34and the motion of transfer link 34 is slaved to the position of rotor 19because of spring 46 and the current or torque constant of motor 16. Sofor a given amount of current, a certain amount of torque out of motor16 produces motion into eccentric pin 54, which in turn produces arelative position of first stage spool 22, and second stage spool 29 isslaved to first stage spool 22 via transfer link 34.

Valve 15 provides a number of advantages. First, motor 16 does not haveto be extremely large. Motor 16 only has to have enough power to movefirst stage spool 22, as first stage valve 21 moves second stage valvespool 29. Second, the first stage valve 21 has a smaller amount ofmotion but amplifies the motion of second stage valve 28 by havingcontrol flow coming from middle lands 68 b and 68 c to the ends 65 a and65 b of spool 29. The slots of the larger second stage spool 29 are muchwider so, with very small motion of first stage spool 22, extreme motionout of second stage spool 29 is achieved. Third, valve 15 results inreduced amounts of leakage. Fourth, the rotor is symmetrical andbalanced. Because of the configuration of the spools, the motor and therotor, acceleration via vibration or shock or some motion that appliesan external force on valve 15 is less likely to move valve 15. Fifth,the manner in which rotor 19 is attached to body 38 makes it easy tonull the electrical current because all that is needed is to move thestator clockwise or counter-clockwise until enough travel of the rotorto meet the desired amplitude is achieved. Sixth, stator 18 is pilotedby the hydraulic chamber which surrounds the valve elements and so themotor is not in as precarious a position and subject to motion that willchange the null. In this design the motor stator can even rotate adegree or two or be shifted and it will not change the null because ofthe constraints.

In the preferred embodiment, rotor 19 is designed to rotate only plus orminus 10 degrees about motor axis 17 off of the rotor null position.Most limited angle torque motors can go up to plus or minus 30 or 35degrees and still have a linear function of torque and current. Thereason this embodiment it is limited to only 10 degrees is it definesthe stiffness of spring 46. The less stroke, the stiffer spring 46becomes, which means the resonant frequency of first stage spool 22increases. The optimum choice is to minimize the rotor angular amplitudeas much as possible but still provide enough that backlash iseliminated.

Various additional changes and modifications may be made to thedescribed embodiments. For example, the size, shape and configuration ofthe various parts are not deemed to be critical, except as may beincorporated in the appended claims. Nor are the materials ofconstruction deemed to be critical. As previously indicated, the valvespools may be slidably mounted directly on the base, or may be slidablymounted within a bushing inserted into a through-bore provided on thebase. In one embodiment, the head of ball end 58 and 59 is split, sothat the rounded head portion consists of two portions that are biasedaway from one another so as to maintain frictionless rolling contactwith the walls of the valve spool seat in which the rounded head isengaged. Alternative motor types may be used to cause the rotor torotate relative to the body.

Therefore, while the presently preferred form of the improved two stageelectrohydraulic valve has been shown and described, and severalmodifications thereof discussed, persons skilled in this art willreadily appreciate that various additional changes and modifications maybe made without departing from the scope of the invention, as definedand differentiated by the claims.

What is claimed is:
 1. A two-stage servovalve comprising: a motor havinga stator and a rotor having a rotor null position and configured andarranged to rotate about a motor axis under the effect of a magneticfield generated by said stator; a bias mechanism configured and arrangedto bias said rotor to said rotor null position; a first stage valvehaving a first valve member movably mounted in a first chamber along afirst valve axis, and adapted to be moved from a first null position toa first off-null position along said first chamber axis to selectivelymeter fluid flow from at least one port defined between said first valvemember and said first chamber; a second stage valve in fluidcommunication with said port of said first valve and having a secondvalve member movably mounted in a second chamber along a second valveaxis, and adapted to be moved from a first position to a second positionalong said second valve axis as a function of movement of said firstvalve member, to selectively meter the flow of fluid from at least oneport between said second valve member and said second chamber; saidfirst stage valve and said second stage valve configured and arrangedsuch that said second stage valve member is at a pressure equilibriumand does not move when said first stage valve member is in said nullposition; a transfer link acting between said first valve member andsaid second valve member; an eccentric drive member acting between saidrotor and said transfer link and having a first eccentric axis that isoff-set a distance from said motor axis and arranged such that selectiverotation of said rotor about said motor axis causes said transfer linkto move; said transfer link and said drive member configured andarranged such that selective movement of said rotor from said rotor nullposition to a second rotor position causes said drive member and saidtransfer link to move said first valve member from said first nullposition to said first off-null position; movement of said first valvemember from said first null position to said first off-null positioncauses said second valve member to move from said first position to saidsecond position; and said movement of said second valve member to saidsecond position causes said transfer link to move said first valvemember from said first off-null position back to said null position. 2.A servovalve as set forth in claim 1, wherein: said transfer linkcomprises a first end portion engaging said first valve member at afirst connection; said transfer link comprises a second end portionengaging said second valve member at a second connection; and saideccentric drive member and said transfer link are coupled at a thirdconnection.
 3. A servovalve as set forth in claim 2, wherein saidtransfer link and said eccentric drive member are rotationally coupledat said third connection.
 4. A servovalve as set forth in claim 2,wherein said transfer link is configured and arranged to move said firstvalve member from said first null position to said first off-nullposition with selective rotation about said second connection
 5. Aservovalve as set forth in claim 2, wherein said transfer link isconfigured and arranged to move said first valve member from said firstoff-null position back to said first null position with selectiverotation about said third connection.
 6. A servovalve as set forth inclaim 2, wherein said transfer link is configured and arranged to movesaid first valve member from said first off-null position back to saidnull position with selective rotation about said first eccentric axis.7. A servovalve as set forth in claim 6, wherein said first eccentricaxis is aligned with said third connection.
 8. A servovalve as set forthin claim 1, wherein: said first stage valve comprises a second port;said second chamber of said second stage valve comprises a first chamberand a second chamber; said first port is flow connected to said firstchamber and said second port is flow connected to said second chamber;and said second valve member is adapted to be moved from said firstposition to said second position along said second valve axis as afunction of a hydraulic pressure differential between said first chamberand said second chamber.
 9. The servovalve as set forth in claim 1,wherein said bias mechanism comprises a torsional spring.
 10. Theservovalve as set forth in claim 1, wherein rotor consists essentiallyof a magnet.
 11. The servovalve as set forth in claim 1, wherein saidstator comprises a circular ring-like core and windings about said coreorientated in opposite directions around said core.
 12. A servovalve asset forth in claim 1, wherein said first chamber and said second chambereach comprise a cylinder and said first stage and said second stagevalve members each comprise a valve spool.
 13. A servovalve as set forthin claim 1, wherein said first stage valve member comprises a slotbounded by substantially-parallel walls and said transfer link comprisesa rounded marginal end portion engaging said slot walls and wherein saidsecond valve member comprises a slot bounded by substantially-parallelwalls and said transfer link comprises a second rounded marginal endportion engaging said slot walls.
 14. A servovalve as set forth in claim1, and further comprising at least one bearing acting between said drivemember and said transfer link.
 15. A servovalve as set forth in claim I,wherein said motor is toroidal.